A step type voltage regulator is a device which is used to maintain a relatively constant voltage level in a power distribution system. Without such a regulator, the voltage level of the power distribution system could fluctuate significantly and cause damage to electrically powered equipment.
A step type voltage regulator can be thought of as having two parts: a transformer assembly and a controller. A conventional step type voltage regulator transformer assembly 102 and its associated controller 106 are shown in FIG. 1. The voltage regulator transformer assembly can be, for example, a Siemens JFR series. The windings and other internal components that form the transformer assembly 102 are mounted in an oil filled tank 108. A tap changing mechanism (not shown) is commonly sealed in a separate chamber in the tank 108.
The various electrical signals generated by the transformer are brought out to a terminal block 110, which is covered with a waterproof housing, and external bushings S, SL, L for access. An indicator 112 is provided so that the position of the tap as well as its minimum and maximum positions can be readily determined.
A cabinet 114 is secured to the tank to mount and protect the voltage regulator controller 106. The cabinet 114 includes a door (not shown) and is sealed in a manner sufficient to protect the voltage regulator controller 106 from the elements. Signals carried between the transformer or tap changing mechanism and the voltage regulator controller 106 are carried via an external conduit 116.
The tap changing mechanism is controlled by the voltage regulator controller 106 based on the controller's program code and programmed configuration parameters. In operation, high voltage signals generated by the transformer assembly 102 are scaled down for reading by the controller 106. These signals are used by the controller 106 to make tap change control decisions in accordance with the configuration parameters and to provide indications of various conditions to an operator.
In order to ensure proper operation, the regulator controller must keep accurate track of the current tap position of the voltage regulator transformer. For example, tap position knowledge is used by the regulator controller for overcurrent operation (sometimes referred to as Vari-amp), systems performance analysis and control, maintenance and safety. For overcurrent operation, tap position knowledge is essential to limit operation of the regulator within acceptable tap position excursions, thereby permitting safe operation of load current outside of the operational maximums as a direct function of tap position.
Tap position knowledge is also a factor in system performance and analysis. This includes the ability to establish statistics on regulator operation such as range and frequency of tap position excursions and associated times and dates. This information may be transferred to a remote location via a communication link.
For maintenance and safety, it is important to place the regulator in the neutral position prior to safe bypass and shutdown. Knowledge of the actual tap position can be used as a fail-safe in conjunction with a neutral position indicator to confirm that the regulator is indeed in the neutral position.
One conventional way to determine tap position is via an electro-mechanical dial that physically attaches to the tap changer mechanism. The electro-mechanical technique has several disadvantages which include high manufacturing cost and inability to communication tap position to a remote location or to the local control without the expense of additional electronic encoded.
Electronic techniques for directly encoding the tap position include the use of digital and analog position encoders. Other indirect means of electronic position encoding that provide a lower cost solution employ various "dead reckoning" methods wherein existing digital and analog signals (e.g. neutral position, tap change command, tap change response, raise/lower command and tap change load current) are used by the controller to derive a tap position.
While "dead reckoning" is lower cost than using an electro-mechanical indicator with an encoder, it is inherently less reliable since it depends on indirect methods to determine position which can cause the tap position to become unknown (lost) or in error.